1. Field of the Invention
One embodiment of the invention relates to data transmissions when data is simultaneously transmitted to a plurality of radio communication devices in the case where a plurality of radio communication devices exist.
2. Description of the Related Art
A multi-user Multiple Input Multiple Output (MIMO) has been recognized as a method for simultaneously performing MIMO communication among radio communication terminals of a plurality of users at the same time and in the same frequency band through evolution of a MIMO technique. While the multi-user MIMO communicates at the same time and in the same frequency band, it is impossible to perform transmissions and receptions at the same time. That is, while the multi-user MIMO may perform a plurality of MIMO transmissions at the same time in the same frequency band, it is presently impossible to simultaneously perform reception. Conversely, while a plurality of MIMO receptions may be performed at the same time and in the same frequency band, it is presently impossible to simultaneously perform transmission. It is impossible to separate transmissions and receptions from each other when they occur at the same time in a terminal. Therefore, in the multi-user MIMO, there is a need to separate transmission times from reception times.
As regards a technique for separating the transmissions from the receptions, for example, a technique of adjusting the timing of transmissions of transmission acknowledgement request frames (Ack Requests) so as to make the reception times of transmission acknowledgement frames (Acks) from a plurality of radio communication devices uniform when a plurality of frames differing in transmission time have been transmitted to the corresponding plurality of radio communication devices is disclosed in U.S. Patent Application Publication No. 2005/0147023. However, since the technique described in this patent document contains the transmissions of the Ack Requests as essential elements, overheads of times for transmitting the Ack Requests are generated, and this generation might lead to deterioration in throughput. In the case in which the Ack Requests are judged as errors owing to a radio environment and may not be received on reception terminal sides, even if transmission data has reached them, there is a need to retransmit the transmission data for the Ack Requests. This retransmission might lead to deterioration in throughput.
As regards another technique for separating the transmissions from the receptions, in the case in which one piece of data is divided and data transmission is performed by using a plurality of transmission channels at the same time, controlling the number of bytes of data frames to be divided to uniform transmission times of the data frames is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-350259. However, the technique described in this patent document does not take any account of combining a plurality of data frames to transmit them when a plurality of data frames are stored in a transmission queue, i.e., transmitting the data frames by a frame aggregation system as described in IEEE 802.11n. In other words, in the technique described in the patent document, it is impossible to combine the transmissions of the data frames with a radio transmission system, such as the frame aggregation system in IEEE 802.11n, at a high rate and with high efficiency. In IEEE 802.11n, by which a standardization proposal is made as a radio communication technique at a high rate by the MIMO technique, a frame aggregation technique for combining a plurality of medium access control (MAC) frames into one physical (PHY) frame to transmit the PHY frame has been proposed as a technique of a MAC layer corresponding to acceleration of the PHY layer (e.g., refer to IEEE 802.11n/D2. 00).